1. Field of the Invention
The invention concerns electrically programmable memories and, notably, the memories known as EPROMs, EEPROMs, FLASH-EEPROMs which correspond to various alternative types of memories for which the programming is done by introducing electrical charges into the floating gate of a floating gate transistor forming the basic element of each elementary cell of the memory.
2. Description of the Prior Art
The programming is electrical. This means that, to program a cell of the memory, this cell is designated by means of a row decoder and, as the case may be, a column decoder and adequate voltages are applied to the designated cell, enabling charges to be injected into the floating gate.
The information stored in the memory is defined by the programmed state of each cell. This programmed state represents a piece of binary information: a cell is programmed or it is not programmed.
To read the information contained in the memory, the programmed state of the cells is examined. For this, a determined cell is addressed by means of the decoder or decoders, and appropriate reading voltages are applied to this cell. This results in an electrical current or a voltage which depends on the programmed state of the cell. By measuring this current or this voltage, it is determined whether the cell has been programmed or not programmed. It is thus possible to collect, cell by cell or cell group by cell group, the pieces of binary information stored in the memory.
More precisely, for example when each memory cell is formed by a floating gate transistor, the reading of the state of the memory cell consists of a comparison between the current coming from the addressed cell and a reference current value. The reference current value is chosen to be substantially in the middle of the interval between the value of current that would be given by a programmed cell (a value close to zero in practice) and the value of current that will be given by a non-programmed cell, with this programmed cell and this non-programmed cell receiving the same read voltages.
Thus, a current comparator, receiving, firstly, the current of the cell to be read (to which the nominal reading voltages are applied) and, secondly, the reference current will flip over unambiguously in one direction or another depending on the programmed state of the cell, thus giving a piece of binary information, at its output, representing the piece of binary information stored in the read cell.
For example, a blank (not programmed) EPROM cell will allow a current of about 200 micro-amperes to flow while a programmed cell will let through only a current of less than 20 micro-amperes under the same reading conditions. The reference chosen may be a current of 100 micro-amperes.
In practice, the current coming from the cell will be preferably converted into a voltage which is a function of this current (for example, by means of an integrator), so that the comparison could be done by a voltage comparator, which is often easier to make than a current comparator. Since the mode of comparison (by voltage or current) is not the object of the invention, and since the making of current/voltage conversions is well known, we shall confine the following explanation to current comparators while bearing in mind the fact that they may actually be voltage comparators. It may be supposed, for example, that the reference value of comparison is defined on the basis of the current coming from a reference blank cell, a current applied to a current/voltage converter for which the gain will define precisely a reference voltage.
Since the physical phenomena brought into play by programming are poorly controlled in industrial conditions, the currents that come from the blank cells and the programmed cells are not known with great precision. They depend on a great many factors including reading voltage values applied to the cells. The current of the programmed cells further depends on the intensity of the programming, namely on the quantity of charges that it has been possible to store in the floating gate of a cell. This quantity of charges depends on the programming voltage and on the period for which this voltage is applied or, again, even on the way in which it has been applied. There is therefore a very great variation between the current values of blank cells and those of programmed cells in a series of memories of one and the same manufactured batch, and even within one and the same memory.
Finally, the programming of a programmed cell gets degraded in the course of time, i.e. the quantity of charges stored in a floating gate diminishes with time especially when the temperature increases (the retention period is of the order of 10 years). The result thereof is that the current coming from a programmed cell gradually increases in the course of time, as and when the threshold voltage of the floating gate transistor gets reduced through this loss of charges.
As a result of these difficulties, it has never been attempted, until now, to envisage the possibility that a piece of information, other than a binary one, could be recorded in a single physical cell of the memory, namely in a single floating gate transistor. However, to store a piece of information other than a single bit, for example a piece of information with three states or a piece of information consisting of two bits, in a single cell, it would be enough to be able to define several voltage or current thresholds, given by the cell in reading mode (instead of a single threshold), so that the position of the current or the voltage with respect to these different thresholds would define the programmed state of the cell among several (more than two) possible states.
It would thus be possible to make great gains in terms of the overall space occupied by the memory, at least for large-capacity memories wherein the size of an elementary cell is the predominant space factor. If a cell stores, for example, two bits of information instead of only one bit, in using the same elementary surface, the overall gain can be assessed at about 25% for the entire memory for one and the same capacity of about one megabit.
This gain in area clearly represents an increase in manufacturing efficiency and, therefore, a reduction in costs.
An object of the present invention is the making of an electrically programmable memory capable of storing, in each memory cell, n possible programmed states with n being at least equal to 3.